Master station of communication system and access control method

ABSTRACT

A communication bandwidth is divided into a beacon period in which all master stations compete for transmission of a beacon packet, a first carrier sense multiple access (CSMA) period in which only authorized specific stations are allowed to compete for access, and a second CSMA period in which all stations are allowed to compete for access. The master stations exchange information with each other about a communication bandwidth being used in the first CSMA period, thereby calculating a communication bandwidth which can be used by each communication system in the first CSMA period, based on the information.

This is a Rule 1.53(b) Divisional of application Ser. No. 11/987,762,filed Dec. 4, 2007 which is a Rule 1.53(b) Divisional of applicationSer. No. 10/912,192, filed Aug. 6, 2004 ,now U.S. Pat. No. 7,315,524.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a master station of a communicationsystem and an access control method, and more particularly, relates toan access control method, which is used in a plurality of communicationsystems sharing the same channel, for preventing interference betweenthe plurality of communication systems from occurring.

2. Description of the Background Art

Conventionally, as a technique for reducing interference between aplurality of communication systems sharing the same channel, thereexists an access control method for reducing the influence ofinterference signals by transmission power control. For example, thereexists Japanese Laid-Open Patent Publication No. 2002-198834 (patentdocument 1), Japanese Laid-Open Patent Publication No. 2003-37556(patent document 2), or Japanese Laid-Open Patent Publication No.2001-53745 (patent document 3).

Patent document 1 discloses a method for attenuating a signal power andan interference power by an attenuator provided in a base station, andcompensating a power of a wireless signal inputted to a receiver with atransmission power of a transmitter of a terminal station so that apower level of the wireless signal becomes a reference level.

Also, patent document 2 discloses a method by which a base station,which has detected interference signals, notifies interferenceinformation to another base station transmitting the interferencesignals via a local communication network for causing the notified basestation to reduce a transmission power based on the interferenceinformation.

Further, patent document 3 discloses the following method. Firstly, afrequency resource is allocated to a wireless station from which highpriority data is to be transmitted. Also, a timing and a frame length inuse for transmission of the high priority data are allocated thereto.When the high priority data is transmitted from an access point (AP) inaccordance with the allocated timing and frame length, the wirelessstation checks channel availability by performing physical carrier sensebefore transmission, and transmits the high priority data only ifchannel availability is verified (i.e., the channel is idle).

However, in the case where the above-described communication system is apower line communication system, depending on the configuration of adevice connected to a network, an amount of signal attenuation in onecommunication system may substantially exceed an amount of signalattenuation which interferes with the other communication systems due tothe characteristics of a power line transmission path. That is, in thecase where patent document 1 or patent document 2 is applied to thepower line communication system and interference between thecommunication systems is performed by power control, there is apossibility that, depending on the device configuration, some devicesmay be unable to perform device communication in the system due toreduced signal intensity. Also, in the case of wireless communications,the similar phenomenon, that is, signal intensity is suddenly reduceddespite the physical closeness, may occur due to attenuation of thesignal intensity, which is caused by a shield.

The above-described conventional configuration does not allow onecommunication system to maintain the quality of communications performedtherein by transmission power control while reducing interference withthe other communication systems. Thus, throughput of each communicationsystem is substantially reduced due to the interference between thecommunication systems, and it is difficult to perform communicationbandwidth control.

Also, in the case where the control disclosed in patent document 3 isemployed, the interference between the communication systems can bereduced by virtual carrier or physical carrier sense. However, it isimpossible to assure quality of service (QoS) of a communicationbandwidth.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a masterstation and an access control method being capable of easily avoidinginterference between communication systems while assuring QoS of acommunication bandwidth of each communication system without performingtransmission power control in a plurality of communication systemssharing the same channel.

The present invention has the following features to attain the objectmentioned above.

The present invention is directed to a master station used in acommunication system in a system environment in which a plurality ofcommunication systems, each of which is composed of at least one slavestation and the master station managing the slave station, shares thesame channel. In order to solve the above problems, the master stationof the present invention includes a communication section, anacquisition section, and a determination section.

The communication section divides a communication bandwidth into abeacon period in which all master stations compete for transmission of abeacon packet, a first carrier sense multiple access (CSMA) period inwhich only authorized specific stations are allowed to compete foraccess, and a second CSMA period in which all stations are allowed tocompete for access, and repeatedly communicates on a periodic basis. Theacquisition section acquires a status of use of communication bandwidthsin the other communication systems. The determination section calculatesa communication bandwidth available in the communication system, towhich the master station belongs, in the first CSMA period based on thestatus of use of the communication bandwidths acquired by theacquisition section, and determines whether communication requested bythe slave station is accepted or rejected in accordance with thecalculated communication bandwidth.

Typically, the beacon packet includes system information providing atleast allocated times of the beacon period, the first CSMA period, andthe second CSMA period. The presence or absence of beacon packettransmission by another master station is checked after a randomback-off process for each cycle of the beacon period. If the absence isconfirmed, the beacon packet is transmitted. On the other hand, if thepresence is confirmed, the beacon packet is not transmitted.

Also, the acquisition section may acquire a status of use ofcommunication bandwidths in the other communication systems byinformation exchange with the other master stations using the secondCSMA period, or may acquire a status of use of communication bandwidthsin the other communication systems from a beacon packet received fromany of the other master stations in the beacon period.

In this case, it is preferable that a total available bandwidth isobtained with respect to a communication bandwidth in the first CSMAperiod based on CSMA access efficiency and a retransmission bandwidth,and that an access request from the slave station is restricted so thatthe communication bandwidth to be calculated by the master station doesnot exceed the total available bandwidth. Also, each of the authorizedspecific stations preferably performs transmission time management inthe first CSMA period so that a transmission bandwidth does not exceed apreviously specified requested bandwidth. Further, an allocated time ATin the first CSMA period may be calculated using an expressionAT=(ΣTn+M)×α, based on communication bandwidths Tn requested in thecommunication systems of the other master stations, a communicationbandwidth M requested in the communication system to which the masterstation belongs, and a predetermined coefficient α.

Also, the beacon packet may include system information providing atleast a start time of the beacon period and a transmission time of thebeacon packet in accordance with a timer value of the master stationtransmitting the beacon packet. In this case, preferably, a transmissiontime of the beacon packet is acquired from the received beacon packet,and a timer value thereof is corrected based on the acquired beaconpacket transmission time. Especially, it is efficient to calculate anintermediate value between a timer value thereof and the transmissiontime of the beacon packet of any of the other master stations, therebycorrecting the timer value to the intermediate value.

The processes performed by each component of the above-described masterstation can be considered as an access control method providing a seriesof procedures. This method is provided in the form of a program causinga computer to execute the series of procedures. This program may beintroduced to the computer via a computer-readable recording medium.Also, each component of the above-described master station may berealized as an LSI, which is an integrated circuit.

As described above, based on the present invention, a communicationbandwidth is divided into the following three periods: a beacon period,a first CSMA period, and a second CSMA period, and an allocation for thefirst CSMA period is determined based on information about a currentlyused communication bandwidth of each communication system. As a result,even if a plurality of communication systems share the same channel, itis possible to easily avoid interference between communication systemsand assure QoS of a communication bandwidth of each communication systemwithout performing transmission power control. Also, different accessmodes do not affect each other.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing an exemplary communication systemenvironment to which the present invention is applied;

FIG. 2 is a block diagram showing an exemplary detailed structure of astation;

FIG. 3 is an illustration for describing period splitting of acommunication bandwidth;

FIG. 4 is a timing chart for describing an access control methodaccording to a first embodiment of the present invention;

FIG. 5 is a flowchart for describing the access control method accordingto the first embodiment of the present invention;

FIG. 6 is an illustration showing the relationship between traffic andthroughput in CSMA access;

FIG. 7 is a sequence for describing a method utilizing a Normal-CSMAperiod;

FIG. 8 is a flowchart for describing the method utilizing a Normal-CSMAperiod;

FIG. 9 is an illustration showing a TXOP in a Controlled-CSMA period;

FIG. 10 is an illustration showing a format of a beacon packet (beaconframe) used for power line communications for storing allocationinformation and announcing the information to the system;

FIG. 11 is an illustration showing the details of a frame controlsection in FIG. 10;

FIG. 12 is an illustration showing the details of a Variant field (VF)in FIG. 11;

FIG. 13 is an illustration showing the details of a segment headersection in FIG. 10;

FIG. 14 is an illustration showing the details of a data body section inFIG. 10;

FIG. 15 is a flowchart showing a procedure of an access control methodaccording to a third embodiment of the present invention (master stationside);

FIG. 16 is a flowchart showing a procedure of an access control methodaccording to the third embodiment of the present invention (slavestation side);

FIG. 17 is an illustration showing information and a timing of a beaconpacket in a beacon period; and

FIG. 18 is an illustration showing an exemplary network system in whichthe access control method of the present invention is applied to ahigh-speed power line transmission.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, with reference to the drawings, embodiments of the presentinvention will be described in detail.

FIG. 1 is an illustration showing an exemplary communication systemenvironment to which the present invention is applied. FIG. 1 shows anexemplary environment including three communication systems 11 to 13which interfere with each other. The communication system 11 includes amaster station 111 and a slave station 112, the communication system 12includes a mater station 121 and slave stations 122 and 123, and thecommunication system 13 includes a master station 131 and slave stations132 and 133.

Each of the master stations and slave stations includes, as shown inFIG. 2, a bandwidth managing section 21, a control section 22, a databuffer section 23, a frame transmitting section 24, a frame receivingsection 25, and a host interface section 26. The bandwidth managingsection 21 manages various information about a communication bandwidth.The control section 22 controls the entirety of the station. The databuffer section 23 temporarily stores various packets. The frametransmitting section 24 transmits the packet stored in the data buffersection 23. The frame receiving section 25 causes the data buffersection 23 to store a received packet. The host interface section 26 is,for example, an interface with a host or an interface with anothermedium (e.g., communication system) such as abridge configuration. Thedetermination section is composed of the bandwidth managing section 21and the control section 22. Also, the acquisition section is composed ofthe data buffer section 23, the frame transmitting section 24, and theframe receiving section 25. Further, a communication section is composedof the control section 22, the frame transmitting section 24, and theframe receiving section 25.

One of the features of the present invention is that a communicationbandwidth used by the communication systems 11 to 13 is previouslydivided into the following three periods: a beacon period, aControlled-CSMA period, and a Normal-CSMA period, each of which has adefined role. During a beacon period, all master stations compete fortransmission of a beacon packet. During a Controlled-CSMA period (firstCSMA period), only authorized specific stations are allowed to competefor access. That is, the Controlled-CSMA period is a carrier sensemultiple access (CSMA) period to which access restriction is applied.During a Normal-CSMA period (second CSMA period), all stations areallowed to compete for access. That is, the Normal-CSMA period is a CSMAperiod to which no access restriction is applied. These three periodsare periodically repeated (see FIG. 3).

The master stations 111, 121, and 131 manage a beacon period, aControlled-CSMA period, and a Normal-CSMA period in accordance with atimer provided in each control section 22, for example. Typically,system information indicating an allocated time of each period istransmitted as information stored in a beacon packet.

Hereinafter, an access control method using the above-described masterand slave stations will be described below.

First Embodiment

FIG. 4 is a timing chart for describing an access control methodaccording to a first embodiment of the present invention. Note that, inthe present embodiment, a case in which start times of the beaconperiods are the same (i.e., start times of the beacon periods arepreviously synchronized) will be described. In order to synchronizestart and end times of the beacon periods, a method which will bedescribed in a third embodiment may be used, for example. Also, assumethat information about a communication bandwidth used by a communicationsystem is transmitted as information stored in a beacon packet. FIG. 5is a flowchart for describing the access control method (bandwidthmanaging method) according to the first embodiment of the presentinvention.

As shown in FIG. 4, each of the control sections 22 included in therespective master stations 111, 121, and 131 performs a random back-offprocess within a beacon period for transmitting its own beacon at astart time of the beacon period. The control sections 22 of the masterstations 111, 121, and 131 perform a random back-off process fortransmitting beacon packets 401, 404, and 407, respectively. When therandom back-off process is completed, each of the control sections 22 ofthe master stations 111, 121, and 131 performs carrier sense in order tocheck (i.e., check a medium) if another beacon packet is beingtransmitted from any of the other master stations, and transmits its ownbeacon packet only if another beacon packet is not being transmittedfrom any of the other master stations. That is, only a master stationwhose random back-off process is first completed can transmit its ownbeacon packet.

In the example as shown in FIG. 4, the master station 121, whichcompletes the random back-off process first, generates the beacon packet404 in the data buffer section 23, and transmits the generated beaconpacket 404 using the frame transmitting section 24. This beacon packetincludes, as system information, a beacon packet transmission time, astart time of a beacon period, a start time of a Controlled-CSMA period,and a start time of a Normal-CSMA period, etc., based on the timer. Notethat the master stations ill and 131 detecting transmission of thebeacon packet 404 by carrier sense stop transmission of the beaconpackets 401 and 407, respectively.

When the beacon packet 404 is received from the master station 121 viathe frame receiving section 25 (step S501), the master stations 111 and131 temporarily store the beacon packet 404 in the data buffer section23. Each of the control sections 22 of the master stations 111 and 131extracts information about a communication bandwidth used by thecommunication system 12 from the stored beacon packet, and stores it inthe bandwidth managing section 21 (step S502). When the new informationis stored in the bandwidth managing section 21, each of the masterstations 111 and 131 determines whether or not a new request isgenerated in each communication system (step S503). In the case where anew request is generated, each of the master stations 111 and 131 newlycalculates a communication bandwidth available in its own communicationsystem based on the stored information, and compares it with the sum ofthe communication bandwidth currently used by its own communicationsystem and a communication bandwidth of the new request (step S504). Asa result of the above comparison, if the sum is smaller than the newlycalculated communication bandwidth, each of the master stations 111 and131 accepts the new request (step S505). On the other hand, if the sumis greater than the newly calculated communication bandwidth, the newrequest is rejected (step S506).

Here, a method performed at step 5504 for calculating a communicationbandwidth available in one communication system based on thecommunication bandwidths used by other communication systems will bedescribed using a specific example. For example, in the case where themaximum bandwidth in the Controlled-CSMA period is 30 Mbps and the sumof the communication bandwidths used by other communication systems is 6Mbps, the efficiency of CSMA is 0.65 and a percentage of a redundantbandwidth (a margin) for retransmission, for example, is 20% based onthe characteristics as shown in FIG. 6. In this case, the totalcommunication bandwidth available in the entirety of the communicationsystems (total available bandwidth) is 15.6 Mbps (=30×0.65×0.8). Thus, acommunication bandwidth available in one communication system is 9.6Mbps (=15.6−6.0). As a result, in this example, a new request isaccepted if a communication bandwidth thereof is equal to or smallerthan 9.6 Mbps.

Other than the above-described method utilizing a beacon period,information about communication bandwidths used by other communicationsystems may be acquired by a method utilizing a Normal-CSMA period. Sucha method will be described using FIGS. 7 and 8.

For example, in the case where the slave station 122 has to assure QoS,the slave station 122 transmits a QoS request packet 611 to the masterstation 121 included in the same communication system (step S801). Themaster station 121, which has received the packet 611 via the framereceiving section 25, temporarily stores the received packet 611 in thedata buffer section 23. Then, the control section 22 of the masterstation 121 transmits status request packets 612 and 614 to the masterstations 111 and 131, respectively, whose presence in its neighborhoodis detected by the beacon packet stored in the data buffer section 23(step S802). Specifically, the control section 22 of the master station121 generates the packets 612 and 614 in the data buffer section 23, andtransmits the generated packets 612 and 614 to the master stations 111and 131, respectively, via the frame transmitting section 24.

Each of the frame receiving sections 25 of the master stations 111 and131, which have received the packets 612 and 614, respectively, storesthe received packet in the data buffer section 23. Then, the controlsections 22 of the master stations 111 and 131 transmit status replypackets 613 and 615 including the information about the currently-usedcommunication bandwidth stored in the bandwidth managing section 21 tothe master station 121, respectively. Specifically, the control sections22 of the master stations 111 and 131 generate the packets 613 and 615,respectively, in the data buffer section 23, and transmit the generatedpackets to the master station 121 via the frame transmitting section 24.

When the packets 613 and 615 are received from the master stations 111and 131, respectively (step S803), the control section 22 of the masterstation 121 determines whether or not the request from the slave station122 is acceptable based on the currently-used communication bandwidthinformation included in the packets, the above-described maximumbandwidth in the Controlled-CSMA period, and the margin (step S804).Then, based on the determination results, the control section 22 of themaster station 121 generates a QoS reply packet 616 indicatingacceptance or rejection of the request in the data buffer section 23,and transmits it to the slave station 122 via the frame transmittingsection 24 (steps S805 and S806).

The control section 22 of the slave station 122, which has received thepacket 616 indicating acceptance of the request, transmits a data packetby a typical CSMA process during a Controlled-CSMA period. That is, atransmitting station transmits a data packet after carrier sense, and areceiving station, which has received the data packet, sends back anacknowledgement packet. In the case where the transmitting stationcannot receive the acknowledgement packet due to a collision or anerror, etc., a random back-off process is performed and a data packet isretransmitted. Specifically, when data such as an IP (Internet Protocol)packet is stored in the data buffer section 23 via the host interfacesection 26, the control section 22 of the slave station 122 determineswhether or not the stored data is QoS data. In the case where the storeddata is QoS data, the control section 22 of the slave station 122verifies that there is no other data frame, etc., by carrier sense afterperforming a random back-off process during a Controlled-CSMA period,and transmits a data frame using the frame transmitting section 24. Notethat, in the case where the stored data is not QoS data, the controlsection 22 of the slave station 122 performs a similar process during aNormal-CSMA period, and transmits a data frame.

Even if the control section 22 of the slave station 122 has many piecesof transmission data, the control section 22 restricts the maximumamount of data transmission in the

Controlled-CSMA period. For example, in the case where a rate iscalculated in system periods, the maximum data transmission amount isrestricted up to 20% greater than the requested bandwidth. Also, asshown in FIG. 9, a restriction is imposed by setting the maximumtransmission opportunity (TXOP), for example. In the case where a TXOPis set as shown in FIG. 9, the control section 22 sequentially transmitsa data packet. In this case, control is performed so that the minimumpacket interval is maintained and other stations cannot performtransmission unless an interval longer than the minimum packet intervalis detected.

Note that the control section 22 may include a typical RTS (Request ToSend) or CTS (Clear To Send) process before transmitting a data packet,whereby it is possible to solve a hidden terminal problem. Also, virtualcarrier duration information, etc., may be included in a packet in orderto perform virtual carrier sense, thereby reducing collision frequency.Also, a master station may transmit a polling packet during aControlled-CSMA period to set a TXOP, whereby a slave station transmitsa data packet in response to the polling. It will be understood that aRTS/CTS sequence may be included. In the case where a data packet, anacknowledgement packet, or a RTS packet is not sent back in response tothe polling, retransmission of a polling packet or polling to a nextstation may be performed.

Second Embodiment

In a second embodiment, with reference to FIGS. 10 to 14, a specificexample of a case in which the access control method described in thefirst embodiment is applied to a power line communication system will bedescribed. FIGS. 10 to 14 are illustrations showing a format of a beaconpacket (beacon frame) used for power line communications for storingallocation information and announcing the information to the system.

Assume that TINF information included in VF (Variant Field) in framecontrol stores information about communication bandwidths T1 to Tn,which are necessary for assuring QoS and accepted by a plurality ofmaster stations 1 to n (n is an arbitrary integer), respectively. Also,as shown in FIG. 14, schedule information SI included in a data bodysection of a beacon packet stores information about an allocated time ofa beacon period, an allocated time AT of a Controlled-CSMA period, andan allocated time of a Normal-CSMA period. In the present embodiment,assume that a beacon cycle is 50 ms.

The master station 111 receives a beacon packet from other masterstations 121 and 131, and stores the received packet in the data buffersection 23. The master station 111 analyzes the beacon packet stored inthe data buffer section 23, extracts TINF information, and stores a setof an address of the master station 121 or 131 and the TINF informationin the bandwidth managing section 21. In the case where there alreadyexists the TINF information, update of the information is performed. Inthe present embodiment, assume that the TINF information of the masterstation 121 is 5 ms, and the TINF information of the master station 131is 8 ms. Note that the TINF information is not updated in the case wherea beacon packet is undetectable due to a collision or an error, etc.Thus, the control section 22 uses the previously received TINFinformation. In this case, it is preferable to set a time period duringwhich the data is valid.

The master station 111 generates a beacon packet in which acommunication bandwidth M requested by its communication system is set.Like in this example, in the case where a communication bandwidthrequest from the slave station 112 belonging to the same communicationsystem is not accepted, the communication bandwidth M is 0 ms. Thus, themaster station 111 generates a beacon packet in which a communicationbandwidth M is set to “0”. The master station 111 determines anallocated time AT of a Controlled-CSMA period using the TINF informationof the master stations 121 and 131 and its own communication bandwidthinformation. For example, in the case where a predetermined coefficientα is 1.3, the allocated time AT of a Controlled-CSMA period iscalculated as follows: (ΣTn+M)×α=(5+8+0)×1.3=16.9 ms, and is set asschedule information SI.

Similarly, each of the master stations 121 and 131 determines anallocated time AT of a Controlled-CSMA period using the TINF informationacquired from other master stations, stores it in the scheduleinformation SI, and generates a beacon. Note that the above-described αcoefficient and expression are illustrative only, and other expressionmay be used in the case where the RTS/CTS sequence or polling is used,for example.

Next, a sequence in the case where a bandwidth request is sent from theslave station 112 to the master station 111 will be described.

Before sending a communication bandwidth request, the control section 22of the slave station 112 sends a test pattern to a communicationdestination station to check the channel conditions therebetween,thereby determining a transmission rate. Specifically, a test pattern isset in the data buffer section 23, and a channel checking frame istransmitted using the frame transmitting section 24. When the channelchecking frame is received, the frame receiving section 25 of thedestination station determines an optimum modulation scheme and anoptimum transmission rate for the channel based on SNR (Signal to NoiseRatio), etc. The control section 22 of the communication destinationstation generates a channel checking result frame based on the abovedetermination results in the data buffer section 23, and transmits it tothe slave station 112. When the channel checking result frame isreceived, the slave station 112 analyzes the frame, and acquires themodulation scheme and the transmission rate necessary for thecommunication.

Assume that the transmission rate is 48 Mbps, for example. In this case,if the slave station 112 has to assure a bandwidth of 6 Mbps, thecontrol section 22 of the slave station 112 generates a bandwidthrequest frame including a transmission rate and a requested bandwidth,and transmits it to the master station 111. In the case where thetransmission rate is smaller than the requested bandwidth, the bandwidthrequest frame is not transmitted since allocation is impossible. In thisexample, a 6 Mbps bandwidth is requested in the transmission rate 48Mbps. Thus, the bandwidth is assured if 6.25 ms is allocated to eachbeacon cycle 50 ms. Note that this calculation may be performed by thecontrol section 22 of the master station 111, or may be performed by thecontrol section 22 of the slave station 112 and is notified to themaster station 111.

When the bandwidth request frame is received, the master station illstores it in the data buffer section 23. The control section 22 of themaster station 111 re-calculates an allocated time AT of aControlled-CSMA period using data in the bandwidth request frame, andobtains AT=25.025 (=1.3×(5+8+6.25). Assume that the upper limit of aControlled-CSMA period is 40 ms. In this case, it is determined that therequest is acceptable since the allocated time AT obtained by the abovecalculation is smaller than 40 ms. Thus, the control section 22 of themaster station 111 generates a request acceptance completion frame, andtransmits it to the slave station 112. In the case where the request isrejected, the control section 22 of the master station 111 transmits arequest rejection frame in a similar manner.

Also, in the case where the request is accepted, the control section 22of the master station 111 updates data in the bandwidth managing section21, sets the TINF information to 6.25 ms, generates a beacon packet inwhich the schedule information SI is updated to 25.025 ms, and transmitsit periodically. When the beacon packet is received, the slave station112 analyzes data of the packet. The slave station 112 acquires theschedule information SI and detects a Controlled-CSMA period, therebytransmitting a data frame requiring bandwidth assurance by using a CSMAprocess during a period of 25.025 ms stored in the schedule informationSI. Note that a data frame requiring no bandwidth assurance istransmitted during a Normal-CSMA period.

Third Embodiment

In a third embodiment, a process for synchronizing start and end timesof the beacon periods, the process being usable in combination with theaccess control method described in the first embodiment, will bedescribed. FIG. 15 is a flowchart showing a procedure of an accesscontrol method according to the third embodiment of the presentinvention (master station side). FIG. 16 is a flowchart showing aprocedure of an access control method according to the third embodimentof the present invention (slave station side).

Each master station individually determines whether or not a beaconpacket is newly received (step S1501). In the case where the beaconpacket is not received, the master station acquires a value of its owntimer (step S1502). The master station determines whether or not theacquired timer value reaches a start time of a beacon period (stepS1503). If a start time is not reached, a process is returned to stepS1501. If a start time is reached, the master station starts a randomback-off process (step S1504).

In the case where a beacon packet is received from another masterstation in the course of the random back-off process (step S1505, YES),the master station acquires a value of its own timer (step S1508). Next,the master station extracts a beacon period start time from the receivedbeacon packet, and adds a predetermined offset time (DelayOffset) tothis beacon period start time, thereby obtaining a beacon packettransmission time in another master station (step S1509). See FIG. 17.Then, the master station calculates an intermediate value between theobtained beacon packet transmission time and its own timer value, andsets the timer to the intermediate value (step S1510). For example, inthe case where a value of a beacon packet transmission time in anothermaster station is “1200 counts” and its own timer value is “1300counts”, a value of the timer is set to “1250 counts”, which is anintermediate value therebetween.

On the other hand, in the case where the random back-off process iscompleted without receiving a beacon packet from any of the other masterstations (step S1506, YES), the master station generates a beacon packetto which a beacon period start time based on its own timer and apredetermined offset time are added, and transmits it to the othermaster stations (step S1507).

Each master station acquires a start time of a beacon period, a starttime of a Controlled-CSMA period, and a start time of a Normal-CSMAperiod, which are included in a beacon packet, and detects a timing.Note that a start time of a next beacon period is acquired by adding aSystemPeriod, which is a generation cycle of a beacon period, to a starttime of the beacon period.

When each slave station receives a beacon packet from a master stationof a communication system to which it belongs (step S1601), the slavestation extracts a beacon packet transmission time from the receivedbeacon packet (step S1602). Then, each slave station sets a timer valueto the extracted beacon packet transmission time (step S1603).

As such, based on the access control method according to the presentinvention, a communication bandwidth is divided into the following threeperiods: a beacon period, a Controlled-CSMA period, and a normal-CSMAperiod, and an allocation for the Controlled-CSMA period is determinedbased on information about a currently-used communication bandwidth ofeach communication system. As a result, even if a plurality ofcommunication systems share the same channel, it is possible to easilyavoid interference between communication systems and assure QoS of acommunication bandwidth of each communication system without performingtransmission power control. Also, different access modes do not affecteach other.

Also, a timer of each station is corrected based on a beacon packettransmission time, whereby it is possible to synchronize the systemswith ease. Especially, a system time of each station is corrected byobtaining an intermediate value between a beacon packet transmissiontime of any of the other stations and its own timer value. Thus, even ifthere may be a station whose timer is way out of sync, it is possible tosynchronize the systems by repeatedly performing a process.

Note that each of the above-described embodiments is realized by a CPUperforming interpretation execution for predetermined program data,which is stored in a storage device (a ROM, a RAM, and a hard disk,etc.) and is executable of the above-described procedure. In this case,the program data may be introduced to the storage device via a recordingmedium, or may be executed directly from the recording medium. Note thatthe recording medium includes a ROM, a RAM, a semiconductor memory suchas a flash memory, a magnetic disk memory such as a flexible disk and ahard disk, an optical disk such as a CD-ROM, a DVD, and a BD, a memorycard, or the like. Also, the recording medium is a concept including acommunication medium such as a telephone line and a carrier line.

Also, the entirety or a portion of the functional blocks composing themaster station of the present invention is realized as an LSI (referredto as an IC, a system LSI, a super LSI, or an ultra LSI, etc., dependingon a degree of integration), which is typically an integrated circuit.Each functional block may be separately constructed in chip form, or maybe constructed in chip form so that a portion or the entire portionthereof is included.

Also, a method of integration is not limited to LSI, and may be realizedby a dedicated circuit or a general purpose processor. Also, an FPGA(Field Programmable Gate Array), which is an LSI that can be programmedafter manufacture, or a reconfigurable processor enabling connectionsand settings of the circuit cells in the LSI to be reconfigurated may beused.

Further, in the case where another integration technology replacing LSIbecomes available due to improvement of a semiconductor technology ordue to the emergence of another technology derived therefrom,integration of the functional blocks may be performed using the abovenew integration technology. For example, biotechnology may be applied tothe above-described integration.

Hereinafter, an example in which the invention described in theabove-described embodiments is applied to an actual network system willbe described. FIG. 18 is an illustration showing an exemplary networksystem in which the present invention is applied to a high-speed powerline transmission. In FIG. 18, a power line is connected to an IEEE 1394interface and a USB interface, etc., provided in a multimedia devicesuch as a personal computer, a DVD recorder, a digital television, and ahome server system via a module having a function of the presentinvention. As a result, it is possible to configure a network systembeing capable of transmit digital data such as multimedia data at highspeed via a power line. This system increases user-friendliness due toreduced cost and easy installability since it is possible to use a powerline, which has already been installed in a home and an office, etc., asa network line without the need for installation of a network cablerequired in a conventional cable LAN.

In the above-described configuration, an example in which an existingdevice is applied to a power line communication via an adapterconverting a signal interface of the existing multimedia device to aninterface of the power line communication has been described. However,it will become possible to perform data transmission between devices viaa power cord of a multimedia device by realizing a multimedia devicehaving a built-in function of the present invention. As shown in FIG.18, it eliminates the need for the adapter, the IEEE 1394 cable, and theUSB cable, whereby wiring becomes simplified. Also, it is possible toconnect to the Internet via a router and connect to a wireless/cable LANusing a hub, etc., whereby a LAN system using the high-speed power linetransmission system of the present invention can be extended. Also, by apower line transmission method, transmission data flows via a powerline, whereby it is possible to eliminate leakage and interception ofdata, which become a problem of a wireless LAN. Thus, the power linetransmission method is effective in protecting data due to improvedsecurity. It will be understood that data transmitted over a power lineis protected by an IPSec, which is an extended IP protocol, encryptionof contents, other DRM scheme, and the like.

As such, it is possible to perform a high-quality power linetransmission of AV contents by realizing a copyright protection functionby the encryption of contents and a QoS function including an effect ofthe present invention (improved throughput and bandwidth allocationresponding flexibly to increased retransmission and trafficfluctuations).

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A master station which manages a terminal station of a communicationsystem, the master station comprising: a receiver operable to receive abeacon packet from another master station which manages another terminalstation of another communication system which shares a same channel withthe communication system, the other master station periodicallyallocating a beacon period and a CSMA(Carrier Sense Multiple Access)period of the other communication system to the channel, the beaconpacket being transmitted from the other master station during the beaconperiod, the beacon packet including information indicating an offsetwhich is a period from a start time of the beacon period to a start timeof the transmission of the beacon packet, the CSMA period of the othercommunication system being a time period during which the masterstations and the terminal stations of the communication system and theother communication system obtain a right of access in competition withone another to perform communication, and a bandwidth manager operableto periodically allocate a beacon period and a CSMA period of thecommunication system to the channel, the beacon period and the CSMAperiod of the communication system being set to be the same as thebeacon period and the CSMA period of the other communication system. 2.A method performed by a master station which manages a terminal stationof a communication system, the method comprising: receiving a beaconpacket from another master station which manages a terminal station ofanother communication system which shares a same channel with thecommunication system, the other master station periodically allocating abeacon period and a CSMA (Carrier Sense Multiple Access) period of theother communication system to the channel, the beacon packet beingtransmitted from the other master station during the beacon period, thebeacon packet including information indicating an offset which is aperiod from a start time of the beacon period to a start time of thetransmission of the beacon packet, the CSMA period of the othercommunication system being a time period during which the masterstations and the terminal stations of the communication system and theother communication system obtain a right of access in competition withone another to perform communication, and periodically allocating abeacon period and a CSMA period of the communication system to thechannel, the beacon period and the CSMA period of the communicationsystem being set to be the same as the beacon period and the CSMA periodof the other communication system.